It is tempting to think of the full as the half done twice: same three sports, double the distance, so roughly double the time and the same way of racing. That mental model is exactly why so many strong athletes fall apart in the back half of an Ironman. A full is not two halves. It is a different race that happens to use the same sports.
Two things change when you double the distance, and the first is straightforward. You cannot hold half-distance intensity for twice as long. A well-paced 70.3 bike sits around 75 to 80 percent of your threshold power, the hardest you could hold for about an hour. A full drops to roughly 65 to 72 percent, and the run ceiling falls with it. That is just the shape of the power-duration curve: the longer you go, the smaller the fraction of your best you can sustain. Most athletes accept this much.
The second change is the one that wrecks races, and it is not linear. The half-marathon you run off a 56-mile bike fades only modestly from your fresh pace. The marathon you run off a 112-mile bike fades far more than twice as much. Double the distance, and you more than double the late-race penalty. The costs compound: glycogen runs low, your gut can only absorb so many carbohydrates per hour across four-plus hours, and every watt you spent over your number on the bike gets withdrawn, with interest, from the marathon. The damage is done by T2. The run just reveals it.
Which is why the classic Ironman blow-up is so predictable. The athlete feels great on the bike, rides it like a long 70.3, rolls into T2 on a personal-best bike split, and then walks the back half of the marathon. They did not run out of fitness. They paced the full like a half. Negative-splitting an Ironman marathon is rare precisely because almost everyone arrives at the run already overdrawn.
The longer clock magnifies everything else, too. Heat you would shrug off in a 70.3 has four-plus hours to cook you. A fueling gap that costs a little in a half becomes a bonk in a full. Time on course is itself a stressor, and the full gives every small error far more room to grow.
So pace by duration, not by distance. On the half, you can afford a higher, steadier push and a run that holds close to your open pace. On the full, the winning move is to under-bike, to leave the bike feeling like you held back, because that restraint is what buys you a marathon you can run instead of survive. The half rewards controlled aggression. The full rewards humility and fueling. Race them the same way, and the full will teach you the difference the hard way.
The takeaways
- A full is not two halves: intensity drops and the late-race fade compounds non-linearly, so it is a different race, not a longer one.
- Bike intensity falls from about 75 to 80 percent of threshold in a 70.3 to about 65 to 72 percent in a full, and the run ceiling drops with it.
- The marathon off a 112-mile bike fades far more than twice as much as the half-marathon off a 56-mile bike, because glycogen, fueling limits, and the over-bike tax all stack on the back end.
- The classic blow-up is pacing the full like a long 70.3: a personal-best bike split, then walking the marathon. The damage is done by T2.
- Pace by duration, not distance: push steadier on the half, deliberately under-bike the full to protect a marathon you can run instead of survive.